Our two vehicles read about 1 MPH slow, one is a 2000 S10, the other a 1989 Corsica. But, the odometer in the car is slower than in the truck, therefore the gas mileage in the car is better than we think or you could say the truck gets slightly worse MPG than we think.
I know that the 91-96 Corsica/Beretta dash printing is off, below 45 mph it is 1.5 mph slow, and above 45 MPH the speed reads 3.5 mph too slow (gps verified), and the two Corsicas I've had in this year range had HIGH reading odometers, as my trip to work in one car was 30.3 miles, and the other was 29.6, where I think the actual milage is somewhere around 29
So we need to use GPS or map out a route to follow (preferably a long one, maybe 50 or more miles) to figure out how accurate, or how inacurate, your odometer is, so that you can adjust your figures so you can figure out your ACTUAL mpg.
If you have GPS to verify it, then just use the GPS as your odometer. If you can't do that, then use the GPS to give you a scale adjustment for your speedometer. If you can't do that, maybe try using the odometer calibration signs they have on certain parts of highways.
If you're not using the stock tire size, you need to scale the odometer for the actual tire size. Automotive engineers refer to the "rolling radius" of the tire, of which the speedometer and odometer are directly driven by. The rolling radius changes with many things -- load transfer to and from the driven wheels under braking and acceleration, the pressure of the air in the tires, and the tread depth, to name a few. The speedo and odo are calbrated linearly to the rolling radius of the stock tire and thus are only as accurate as the rolling radius is constant. Unless you're using a GPS odometer (which I don't know about their accuracy) I would say it's unreasonable to expect better than 3-4 percent accuracy from any speedometer.
Even your highest reading odometer at 30.3 miles is only about 4.5% off of the "actual" distance, if that is indeed the actual distance. If you actually get 25mpg, you will be reading 26.1mpg. The measurement system is really not accurate enough to do better than that.
On the flip side, the measurement system for gasoline use (the pump) is really accurate to a thousandth of a gallon. The gas companies invest a lot to make sure they don't give you too much gas.